Tag: Dear Abbe

DEAR ABBE: I was cruising around the internet the other day in my web-rocket when I came across this stellar image of the comet ISON, taken by the Hubble space telescope. The stars appear to be twinkling. I was under the impression that the twinkling effect we see on earth is due to the atmosphere, and last time I checked the Hubble was something of a space telescope, so shouldn’t Hubble be above twinkling? -HUMBLED BY HUBBLE

DEAR HUMBLED: You’re right about twinkling, it is not apparent to observers located outside of a dense atmosphere, the topic of the 1969 paper “Importance of observation that stars don’t twinkle outside the earth’s atmosphere” by astronaut Walt Cunningham and co-author L. Marshall Libby. But twinkling is not likely to produce such picturesque points on stars as you see in that Hubble image. Rather, what appears to the naked eye as twinkling will serve to blur and smudge the image of a star in a time-averaged intensity measurement, such as a photograph.

The spikes you see in the image in question are due to something else entirely. Twinkling stars are a result of a fickle refractive media, the atmosphere, inadvertently being included in an imaging system. The culprits causing these spikes are intentionally built into the optical system, though the effect on the image formed is a byproduct of their form rather than their primary function. What you see as four regular points oriented to the same direction on every bright star is actually the result of diffraction around the secondary mirror support struts[2][3]. Since the spikes are the Fourier transform of the struts themselves[4], they will affect every light source in the image according to their shape and brightness. The appearance of diffraction spikes is so common that the human mind essentially expects it in this type of image, and can be considered aesthetic. Ultimately, though, any light ending up in the diffraction spikes is light that could have contributed to forming the accurate image of the scene. If a dim object of interest resides by a very bright point of light, the diffraction spikes of the latter can interfere with the clear few of the dim object.

Hubble’s successor, the James Webb telescope will have three struts rather than four[5], resulting in a very different set of diffraction spikes. Not only will the James Webb struts differ in number, but these will be arranged in a sort of triangular pyramid. Diffraction around the strut will affect the final image differently at different lengths along each strut, because they will occupy a range of distances from the primary mirror. The resulting spikes should be quite interesting.